Radio-frequency Integrated Circuits (RF-ICs) are widely used in the field of telecommunications, as they form an essential constituent of the signal transmission/reception block. High-frequency or RF inductors are one of the key elements for signal processing, particularly in radio frequency analog circuits, such as filters, impedance-matching circuits, amplifiers and transformers. Inductor elements in RF-ICs are made of miniature metal loops fabricated through well-known lithographic processes.
In general, RF inductors consume a large fraction of real estate in IC chips and, therefore, it is very desirable to achieve the maximum possible level of compactness and efficiency in their design and fabrication. Presently, a higher value of inductance per unit area (inductance density) is the preferred characteristic of RF inductors. In order to achieve this characteristic, a suitable magnetic material is to be used as the inductor core. Thus, inductor loops may be enclosed in a suitable magnetic core material to enhance the inductance of the metal loop inductor. This approach obviates the need for a substantial redesign of inductor geometry, which would otherwise be necessary to increase inductance density. Any alteration of device geometry eventually leads to fabrication complexity or uncontrollable parasitic effects or both.
Magnetic alloys, such as Permalloy and a wide range of compositions in the amorphous Co—Zr—Nb-alloy system are potential candidates for the inductor core required to enhance inductance/area (inductance density). Thin films of these materials, prepared by various methods have been studied for their magnetic behaviour in the frequency range from a few MHz to a few GHz.
However, it has been recognized that the negative effect of power loss in the core made of such electrically conducting alloys more than offsets the positive effect of their high permeability to the performance of a circuit operating at GHz frequencies. This has prompted investigations into the use of a magnetic material with very high electrical resistivity as the inductor core, rather than metallic alloys like N—Fe and Co—Zr—Nb.
Ferrites are known for their magnetic and electrical properties (including high magnetization density and high electrical resistivity) and have been used in high frequency circuits as the magnetic core. However, in view of the miniaturization of such devices and the development of on-chip planar inductor structures, the available technology of ferrite film fabrication has not been found to be adequate to meet the high frequency operations. One of primary reasons for this inadequacy is the high-temperature processing required for the formation of crystalline ferrites such as thin ferrite films that are normally required in RF-ICs, which is incompatible with today's Complementary metal-oxide-semiconductor (CMOS) technology with its aggressive device feature sizes.
Alternately, the RF magnetron sputtering route is employed to deposit ferrite films, on such devices. However, in this method, high temperature post-deposition processing is required, which is not compatible with presently-used CMOS RF-ICs having aggressive geometries.
Electron Cyclotron Resonance (ECR) microwave plasma-enabled sputtering, which is a modified sputtering method meant to reduce the processing temperature of the ferrite film deposition, is also employed. However, the relatively low deposition rate of the process imposes a major deterrent, as it raises fabrication cost significantly.
Pulsed laser deposition (PLD) is another method that can form ferrite films at temperatures that may be low enough to be compatible with today's CMOS technology. However, this method is associated with scalability issues, making it incompatible with large-scale device fabrication.
Spin-spray plating of ferrite is another known method devised to meet the need for a low-temperature process for ferrite film formation. While scalable, the chemistry of the process requires an oxidizing agent such as hydrogen peroxide (H2O2) and a pH modifier, preferably a base, making the method incompatible with Si substrates and with lithographed metal coils forming the miniature inductors in RF-ICs. However, achieving sufficient and strong adherence of the resulting ferrite film to the substrate, using this process, has become a significant challenge.
In view of the non-uniformity of the grain size of a ferrite material that usually results from the different processes listed above used to incorporate (in the form of a thin film) in a device such as an inductor, the result is often a material having an undesirably high degree of porosity or a low degree of flatness or both. Porosity reduces inductance per unit area that can be attained in RF-CMOS circuit, where “chip real estate” is precious. A low degree of film flatness generally makes IC processing more expensive.
In addition, when a desirable ferrite composition, namely ZnFe2O4, attains a grain size of about 50 nm and above due to being processed at elevated temperatures and under conditions of thermodynamic equilibrium, it becomes an anti-ferromagnet at low temperatures and possesses low permeability at room temperature. Therefore, such a material or composition has a limited use as a magnetic core material in RF-ICs.
Magnetic inductor cores made of metal alloys such as N—Fe and Co—Zr—Nb, when used as inductor core in RF-CMOS systems, suffer due to electrical losses at higher frequencies because of their low electrical resistance.
In a known microwave-assisted process to prepare a nanostructured ferrite film composition, a mixture of alcohols is employed as a solvent. However, such a process is faced with a problem of agglomeration of ferrite grains resulting in a non-uniform grain size of the ferrite grains.
In addition, the materials thus prepared using known processes require post-deposition treatment, such as annealing at elevated temperatures, thus making the IC fabrication process CMOS-incompatible.
The energy consumed in processes that use vacuum-based techniques that are used to deposit thin films on a substrate is high, since the creation and sustenance of the required vacuum levels is very energy-intensive and thus expensive as well.
Gardner. D et. al., in Review of On-chip Inductor Structures with Magnetic Films IEEE Transactions on Magnetics 45, 4766 (2009), discloses a series of magnetic inductor structures capable of operating frequencies up to 1 Ghz.
U.S. Pat. No. 7,438,946 and JP61-030674 disclose a method of depositing ferrite material on a substrate at a low temperature.